1. Field of the Invention
The present invention relates to a surface acoustic wave device using a shear horizontal type ("SH-type") surface acoustic wave and including a plurality of surface acoustic wave elements connected in such a manner to form, for example, a ladder-type filter.
2. Description of the Related Art
There are many conventional types of surface acoustic wave (SAW) devices which include a plurality of SAW elements. For example, a SAW filter in which a plurality of SAW resonators are arranged to define a ladder circuit is known and referred to as a ladder filter. FIG. 1A shows a conventional ladder filter 201 disclosed in Japanese Laid-open patent application No. 5-183380, and FIG. 1B is an equivalent circuit thereof. The ladder filter 201 includes a piezoelectric substrate 202 and series one-port SAW resonators 203 and 204 and parallel one-port SAW resonators 205, 206 and 207 disposed on the piezoelectric substrate 201. The series one-port SAW resonators 203 and 204 are connected in series between an input terminal IN and an output terminal OUT to define a series arm, and parallel one-port SAW resonators 205-207 are respectively connected in parallel between the series arm and a ground potential to each define a parallel arm.
As shown in FIG. 1A, the SAW resonators 203-207 each includes a respective pair of interdigital transducers (IDTs) 203a-207a and a respective pair of grating reflectors 203b-207b disposed on opposite sides thereof. In the SAW resonators 203-207, surface acoustic waves excited by the IDTs 203a-207a are confined between the grating reflectors 203b-207b so as to form standing waves. Each of the resonators 203-207 has a resonance characteristic in which the impedance of the resonator is low in the vicinity of a resonant frequency and the impedance is high in the vicinity of an antiresonant frequency.
In the ladder filter 201, the resonant frequency of series one-port resonators 203 and 204 are constructed to be coincident with the antiresonant frequency of the parallel one-port resonators 205-207. Thus, the ladder filter 201 is provided with a passband defined by the antiresonant frequency of the series one-port resonators 203 and 204 and the resonant frequency of the parallel one-port resonators 205-207.
The conventional ladder filter 201, and other ladder filters of this type have been widely applied to a televisions, VCRs, communication devices such as a cellular phone or the like. However, there has been a continuous demand for improving the performance of such a ladder filter and miniaturizing such a ladder filter.